Experience to date with Indium-111 labeled autologous platelets in the dog-catheter model has demonstrated excellent thrombus labeling and scintigraphic imaging of recent thrombus, considerably less platelet uptake for thrombus established at 2, 24 hours and 5-7 days previously. This is of great concern for clinical use of the technique, where pre-existence of thrombus is almost invariable, however surgical experience has shown that acute thrombus is continuously being added to chronic clots in many locations. Frequently acute clinical embolic and thrombotic manifestation of activity occur in long standing lesions notably peripheral venous thrombosis and left ventricular aneurysms. Point-source phantom studies have demonstrated minimum imageability of microcurie 111 in a tissue-equivalent radioactive phantom at 2.5 cm depth, and of 0.5 microcurie at 12.5 cm depth, suggesting further limitation for this technique. Positive clinical images have been seen in vivo in patients with intravascular thrombosis associated with intraluminal tumor extension. Clinical studies are progressing and further evaluation of the technique in various clinical states will continue. These include extracranial cerebrovascular atherosclerosis, acute myocardial infarction, deep venous thrombosis with or without pulmonary embolism; cardiac and other vascular prosthetic devices, renal transplants with acute rejection, and intravascular catheterization. Our research activities during the next funding period will be directed towards evaluating these areas.